Carpenter's tape measures typically incorporate a spring-loaded rolled tape in a housing. When used, the tape or "blade" is extracted from the housing to the desired length for measurement. Most tape measures presently contain a mechanism for locking the measuring tape into a fixed position when it is extended at the desired length. These locking mechanisms have usually either a "slide" mechanism or a "toggle-type" mechanism.
In a typical "slide" mechanism, a button slides along a fixed path between a locked position and an unlocked position. As the button slides to the locked position, a lock shoe is gradually brought into contact with the tape measure blade. When the button is slid all the way into the locked position, the contact between the lock shoe and the blade is sufficient to press the blade against a portion of the tape measure housing, thus locking the blade in place at the desired length. The blade is then unlocked by sliding the button back to the unlocked position.
Slide lock mechanisms have presented several disadvantages. First, guide paths must be formed in the tape measure housing to guide the sliding button from position to position. Sliding tabs on each end of the button require recesses within the housing into which they can slide as the button is pushed in either direction. Providing such recesses requires a larger housing which is harder to grasp. Second, moving the sliding button is often awkward. It is difficult in many instances to hold the spring-loaded blade in its extended position while pushing the button down in the proper direction, since little leverage can be obtained without changing the position of the hand. Third, movement of the sliding button is not determinative of the position of the lock shoe with respect to the blade. To lock the blade, the sliding button must be moved far enough to cause sufficient contact by the lock shoe. However, it is often difficult to determine whether the button has been moved sufficiently. As a result, it is also difficult to determine whether the blade is locked in place without releasing either the blade or the hand holding the housing. Fourth, it is also difficult to determine where the finger or thumb is positioned with respect to the button by simply touching the button. The surface of the button must be felt to determine the position of the finger on the button before the user can be sure that movement of the finger will move the button in the desired direction. Finally, the lock shoe in sliding mechanisms usually meets the tape blade at less than the perpendicular and can push the blade a small distance back into the housing as the lock is engaged, thus changing the desired position of the blade. Typical slide mechanisms are disclosed in U.S. Pat. Nos. 3,689,004 and 3,716,201. Commercially available products using a sliding mechanism include Product No. 33-425 (available from Stanley Tools).
FIGS. 1 and 2 depict typical "toggle-type" locking mechanisms used in prior devices. One type of lock, as shown in FIG. 1, is movable between a locked and lock position. Once the leading end 12 of measuring tape 11 is extended out of housing 10 to a desired length, it can be locked in place by lock shoe 15 engaging the tape. Lock shoe 15 is slidably mounted within housing 10 between two constraining walls 32. Button 16 is mounted within housing 10 for rotation about button pivot 19 positioned approximately in the center of button 16. A cam surface 16a on button 16 is adapted for contacting the upper edge of lock shoe 15 and causing lock shoe 15 to engage the upper surface of measuring tape 11 to lock the tape in place when moved to a locked position.
In an "over center" lock, shown in FIG. 2, Button 16 and lock shoe 15 are connected at a notch 18 such that movement of button 16 moves lock shoe 15. During such movement, the point of button-lock shoe contact passes across the imaginary line between button pivot 19 and the point of lock shoe-measuring tape contact (i.e., the button-lock shoe contact point passes "over center") when moved to a locked position. Commercially available products incorporating these or similar mechanisms include Product No. Lufkin Y125 and 8425 (available from Coopertools Division, Cooper Industries).
Both toggle-type mechanisms are locked and unlocked by "toggling" button 16 between different positions by applying force to button 16 at two different points at the ends of the button, which causes button 16 to rotate about button pivot 19 in two different directions. To lock the mechanism, force is applied to point 33 on button 16, thus rotating button 16 about button pivot 19 in a clockwise direction (as shown in FIGS. 1 and 2). As button 16 rotates, lock shoe 15 is pressed against the upper surface of measuring tape 11, locking it in place. To unlock the mechanism, force is applied to a different point 34 on the opposite end of button 16, thus rotating button 16 about button pivot 19 in the opposite direction (counterclockwise in FIGS. 1 and 2).
Although toggle-type locking mechanisms in general perform better than sliding mechanisms, they also exhibit certain disadvantages. The most significant disadvantages are ergonomic disadvantages caused by the need to apply force at different points and in different directions. This arrangement requires awkward positioning and movement of the user's hand, particularly the thumb, to lock and unlock the device. Locking and unlocking may also require repositioning of the user's hand to contact the different points on the button or changing the user's grip to apply the necessary opposite forces. In use, such changes in position can cause the tape measure to move from the desired alignment with the object being measured, requiring repositioning of the tape measure to assure accurate measurement. Repositioning of the hand may be especially disadvantageous where the tape measure is used in tight spaces where repositioning of the hand may not be possible. Efficient operation of such a mechanism, like the sliding button mechanism, also requires that the user determine where a finger is located on the surface of the button. Placement of a finger on the button does not alone determine whether the mechanism can be activated in the desired fashion by movement of the finger without determining the finger's position on the button.